The present invention relates to a method for electrolessly depositing gold onto a palladium substrate in a uniform manner. More particularly, the present invention relates to a method for activating a palladium substrate prior to electroless gold deposition, to serve as an intermediate barrier layer between the electroless deposited gold and another metal such as nickel to reduce diffusion of such underlying metal into the gold grain boundaries.
Electroless plating is useful in applications requiring coatings for complex shapes. This capacity makes electroless plating techniques particularly useful in the electronics industry, for example, in the metallization of conductors and insulators in printed circuit boards. Base metals such as nickel and copper are often used in electroless plating processes to metallize conductors and insulators. However, because of its low contact resistance and beneficial effects on bonding leads, electroless gold plating is continually desired for coating complex shapes and electrically isolated tracks and bonding pads in printed circuit boards. It has been found, however, that electroless gold plating baths are not as easy to operate as the electroless base metal plating baths, for example, with respect to bath stability and plating rate.
The term "autocatalytic" is used herein to describe a plating system which is capable of depositing gold on a gold substrate. Autocatalytic gold plating is advantageous, for example, for increasing the thickness of existing gold surfaces which are too thin for some uses. Autocatalytic electroless gold plating compositions are known in the art. Reference may be made, for example, to U.S. Pat. Nos. 3,700,469; 3,917,885; and 4,337,091.
U.S. Pat No. 3,700,469 to Okinaka discloses an autocatalytic electroless gold plating bath containing a soluble gold cyanide complex ion, excess free cyanide to stabilize the gold cyanide complex ion, an alkaline agent as a pH adjustor, and an alkali metal borohydride or dimethylamine borane as a reducing agent. Although a truly autocatalytic plating bath, the Okinaka bath has several limitations including instability, low plating rate (about 1 micron per hour), difficulty of bath replenishment, and sensitivity to nickel ions in solution.
U.S. Pat. No. 3,917,885 Baker discloses an autocatalytic electroless plating bath purportedly having improved stability by adding to the bath an alkali metal imide complex of the metal to be plated, e.g., gold. However, the Baker bath has been found to have the same problems as the Okinaka bath, especially sensitivity to nickel contamination and deteriorating plating rate.
U.S. Pat. No. 4,337,091 to El-Shazly et al discloses the use of trivalent gold metal complexes as the source of gold in an electroless gold plating bath, the reducing agent being any of the borohydrides, cyanoborohydrides or amine boranes that are soluble and stable in aqueous solution. A later version of the El-Shazly bath, disclosed in U.K. Patent Application G.B. No. 21/21/444A, uses a mixture of trivalent and monovalent water-soluble gold cyanide complexes. The El-Shazly baths suffer from the same limitations as the prior art baths described above.
In autocatalytic plating, oxidation/reduction reactions begin simultaneously when the substrate is immersed in the plating bath. These reactions occur at the surface of the metal or metallized substrate. At the substrate, the gold ions accept electrons from the reducing agent and deposit a gold film on the substrate. Initially, the reducing agent reacts on the substrate, giving electrons to the metal ions and being converted to its oxidized form. The gold film then catalyzes the reaction and causes it to continue autocatalytically.
In an electroless bath containing an alkali metal gold cyanide complex, an alkali metal cyanide complexing agent, an alkali metal hydroxide, and dimethylamine borane as a reducing agent, cyanide is a strong poison for the oxidation of the reducing agent. However, cyanide is necessary to prevent the spontaneous decomposition of the bath as shown below: EQU (CH.sub.3).sub.2 NHBH.sub.3 +4OH.sup.- +3Au(CN).sub.2.sup.- =(CH.sub.3).sub.2 NH+BO.sub.2.sup.- +1.5 H.sub.2 +2H.sub.2 O+3Au+6CN.sup.-
Although electroless gold plating is continually desired for coating complex shapes and useful in bonding pads and printed circuit boards, base metals such as nickel and copper which are often used in electroless plating processes often contaminate the gold surface as a result of intermetallic diffusion. Problems in joining, for example, soldering, thermal compression bonding or ultrasonic bonding and the alteration of the surface resistivity of the gold can result. The problem of diffusion was investigated by Paul et al, as shown in Thin Solid Films 53 (1978) 175-182. Conductive multilayers of metals having an underlying base metal and intermediate protective layer and an outer conductive layer such as gold were studied. Although various techniques have been used to reduce surface diffusion between an underlying metal substrate such as nickel or titanium and an outer conductive substrate such as gold, it has been found that most electroless gold plating compositions either did not plate at all onto barrier layers such as palladium, or plated sporadically with inconsistent incubation periods. In fact, many baths use palladium electrodes to measure the potential of the bath, since gold does not plate onto the palladium. It would be desirable therefore to be able to provide a procedure for activating palladium to allow uniform electroless gold deposition onto the palladium substrate.